Computational and experimental analysis of the use of lower-limb prostheses : concerning comfort, structural design and gait performance

Pao Yue-kong Library Electronic Theses Database

Computational and experimental analysis of the use of lower-limb prostheses : concerning comfort, structural design and gait performance


Author: Lee, Chiu-chun Winson
Title: Computational and experimental analysis of the use of lower-limb prostheses : concerning comfort, structural design and gait performance
Degree: Ph.D.
Year: 2006
Subject: Hong Kong Polytechnic University -- Dissertations
Prosthesis -- Design and construction -- Data processing
Leg -- Amputation
Department: Dept. of Health Technology and Informatics
Pages: xxv, 241 leaves : ill. ; 30 cm
Language: English
InnoPac Record:
Abstract: This thesis involves the study of prosthetic socket-residual limb interface stress, pain-stress relationship, prosthetic design and strength as well as walking patterns of trans-tibial amputees. Although each of these appears to be a stand-alone study, a good systematic understanding of these areas is crucial for the improvement of comfort, walking performance and safety - determinants of the quality of a lower-limb prosthesis. This thesis focuses on one type of trans-tibial prosthesis- monolimb- having the socket and shank molded into one piece of thermoplastic. Due to the flexibility of thermoplastics, the shank of a monolimb can deflect during walking. If properly designed, monolimbs may compensate for the lost ankle motions through the shank elasticity, meanwhile lowering the weight and cost. To this end, systematic design of monolimbs making good use of the elastic shank was performed. The effects of shank flexibility of monolimbs on socket-limb interface stress and gait performance were evaluated. In an attempt to relate the socket-limb interface pressure to comfort, the pressure tolerant ability of the residual limb was studied. To evaluate the pressure-pain relationship, pressure was applied through a small indenter to the residual limb until pain was perceived. The site-dependence of the pressure-pain relationship, and the effect of walking and interface material on the ability to tolerate pressure were studied. Results showed that contrary to common beliefs, regions with a thicker layer of soft tissue such as mid shaft of fibular had even lower load-tolerant ability than regions with thin skin such as tibial tuberosity. Amputees tended to tolerate higher average pressure with softer indenting materials, and after walking with the prostheses. A finite element (FE) model was built to study the pressure distribution and soft tissue distortion under the indenter. The model suggested pain is triggered when peak pressure applied to the limb surface exceeds a certain limit. The next important step towards improvement of prosthesis fit is to study the socket-limb interface stress. Over the past decades, FE models have been used to evaluate the socket-limb interface stress under different simulated conditions. To simplify the problem, however, assumptions such as no pre-stress induced in the limb tissues within a rectified socket, adhesion at the limb-socket interface, and negligible inertia loading were often made. This thesis presents a new approach in the FE modeling of the contact interface between the limb and socket, with consideration of pre-stresses and frictional slip. With this model, the effects of pre-stress and inertia load on interface stress prediction were revealed. Experimental pressure measurements were performed and compared to the FE models. The pressures predicted by the FE models were in the range of the clinical measurements. Structural integrity of a prosthesis is equally important to comfort. To study the mechanical interaction between the limb and socket as well as the stress distribution within an entire prosthesis, the limb-socket FE model was expanded incorporating a shank and a prosthetic foot. The shank and the socket were modeled as one piece of thermoplastic material (monolimb). Different shank geometries were used. Loading was applied to simulated stance phase of the gait. Results suggested that a flexible shank tended to lower the stress applied to the limb. Predictions of possible structural failures based on the calculated von Mises stress as well as the degree of shank deflections were performed. A systematic method is needed to optimize and evaluate the shank design of a monolimb, concerning the dilemma between shank flexibility and structural strength as well as the function of a flexible-shank monolimb. To suggest a monolimb design with appropriate shank flexibility and adequate structural strength for normal use, FE analysis was used to simulate structural tests on monolimbs of different designs, and the Taguchi method was employed to identify the most effective factor to control the deformation and stress. By fine-tuning the design factors, an optimized flexible elliptical-shank monolimb was suggested. Experimental validation showed a good match with the theoretical result. Fatigue test was performed to document the structural changes of the monolimb upon persistent use. At the end, the gait patterns and the subjective feedback of trans-tibial amputee subjects using the optimized monolimb was studied. Results suggested the optimized monolimb can potentially offer similar functional advantages to the high-cost prosthetic feet.

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